Mixed-potential-type sensor

ABSTRACT

A mixed-potential-type sensor for measuring the concentration of nitrogen oxide contained in a gas under measurement including a solid electrolyte layer having oxygen-ion conductivity, and a pair of porous electrodes formed thereon. One electrode is covered with a first layer containing tungsten oxide as a main component. The other electrode is covered with a gas impermeable second layer. The second layer is in contact with the other electrode without intervention of the tungsten oxide component. The solid electrolyte layer is porous and allows the gas under measurement to permeate from an externally exposed surface of the solid electrolyte layer to the other electrode. The concentration of the nitrogen oxide is detected from a potential difference developed between the electrodes.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a mixed-potential-type sensor fordetecting the concentration of nitrogen oxide (NOx).

Description of the Related Art

Environmental control, process control, etc., requires measurement ofthe concentration of NOx contained in a gas under measurement. Inparticular, diagnosis of asthma requires measurement of NOx contained inexhaled air at a very low concentration (several ppb to several hundredsppb).

In view of these requirements, a technique has been proposed ofconnecting, in series, a plurality of sensors each including a referenceelectrode and a sensor electrode (detection electrode), and forming thesenor electrode using WO₃ so as to enhance the selectivity to NOx (seeJapanese Kohyo (PCT) Patent Publication No. 2010-519514 (claim 7)). Seealso U.S. Patent Application Publication No. 2015/0250408, incorporatedherein by reference in its entirety.

Since WO₃ eliminates the catalytic activity of the electrode forconverting NO₂ to NO, a potential difference is developed between adetection electrode containing WO₃ and a reference electrode containingno WO₃. Thus, the selectivity to NOx is enhanced.

Incidentally, the manufacture of a detection electrode containing WO₃poses a problem that in a firing step, WO₃ sublimates (scatters) andadheres to the surface of the reference electrode. If WO₃ adheres to thesurface of the reference electrode, the reference electrode also losesits catalytic activity. As a result, the potential difference developedbetween the reference electrode and the detection electrode decreases,and the NOx detection sensitivity of the sensor is lowered. Therefore,sensitivity may vary among the plurality of sensors.

SUMMARY OF THE INVENTION

In view of the above-described problems, an object of the presentinvention is to provide a mixed-potential-type sensor which includes adetection electrode containing tungsten oxide, and which preventsdeterioration of the sensitivity in detecting nitrogen oxide.

The above object has been achieved by providing, in a first aspect (1),a mixed-potential-type sensor for detecting the concentration ofnitrogen oxide contained in a gas under measurement. Themixed-potential-type sensor comprises a solid electrolyte layer havingoxygen-ion conductivity; and a pair of porous electrodes formed on thesolid electrolyte layer, wherein one of the porous electrodes is coveredwith a first layer containing tungsten oxide as a main component, theother of the porous electrodes is covered with a gas impermeable secondlayer, the second layer is in contact with the other of the porouselectrodes without intervention of a tungsten oxide component, the solidelectrolyte layer is porous and allows the gas under measurement topermeate from an externally exposed surface of the solid electrolytelayer to the other of the porous electrodes, and the concentration ofthe nitrogen oxide contained in the gas under measurement is detectedfrom a potential difference developed between the porous electrodes.

According to the mixed-potential-type sensor (1), when this sensor ismanufactured by firing or the like, the tungsten oxide contained in thefirst layer diffuses into the one of the porous electrodes, and reachesthe interface between the one porous electrode and the solid electrolytelayer. As a result, the one porous electrode loses its catalyticactivity for converting NO₂ to NO and functions as a detectionelectrode.

Meanwhile, even when the tungsten oxide contained in the first layersublimates due to firing, since the second layer is gas impermeable, thetungsten oxide component cannot reach the interface between the secondlayer and the other of the porous electrodes, and the tungsten oxidecomponent is not present at the interface. Therefore, the other of theporous electrodes has a catalytic activity for converting NO₂ to NO andfunctions as a reference electrode.

As described above, the second layer prevents the other of the porouselectrodes from losing its catalytic activity as a reference electrode.Therefore, a potential difference is reliably developed between the oneof the porous electrodes which serves as a detection electrode and theother of the porous electrodes which serves as a reference electrode.Thus, deterioration of the sensitivity in detecting nitrogen oxide canbe prevented.

In a second aspect (2), the above object has been achieved by providinga mixed-potential-type sensor for detecting the concentration ofnitrogen oxide contained in a gas under measurement. Themixed-potential-type sensor comprises a solid electrolyte layer havingoxygen-ion conductivity; and a pair of porous electrodes formed on thesolid electrolyte layer, wherein one of the porous electrodes is coveredwith a first layer containing tungsten oxide as a main component, theother of the porous electrodes is covered with a second layer whichcaptures a tungsten oxide component originating from the first layer,the solid electrolyte layer is porous and allows the gas undermeasurement to permeate from an externally exposed surface of the solidelectrolyte layer to the other of the porous electrodes, and theconcentration of the nitrogen oxide contained in the gas undermeasurement is detected from a potential difference developed betweenthe porous electrodes.

According to the mixed-potential-type sensor (2), even in the case wherethe tungsten oxide contained in the first layer sublimates when thissensor is manufactured by firing or the like, the second layer capturesthe tungsten oxide component originating from the first layer.Therefore, the tungsten oxide component cannot reach the interfacebetween the second layer and the other of the porous electrodes, and thetungsten oxide component is not present at the interface. Therefore, theother of the porous electrodes has a catalytic activity for convertingNO₂ to NO and functions as a reference electrode.

As described above, the second layer prevents the other of the porouselectrodes from losing its catalytic activity as a reference electrode.Therefore, a potential difference is reliably developed between the oneof the porous electrodes which serves as a detection electrode and theother of the porous electrodes which serves as a reference electrode.Thus, deterioration of the sensitivity in detecting nitrogen oxide canbe prevented.

In a preferred embodiment (3) of the mixed-potential-type sensoraccording to the first aspect (1) of the invention, the second layercontains SiO₂ as a main component.

According to the mixed-potential-type sensor (3), the second layerbecomes gas impermeable without fail.

In another preferred embodiment (4) of the mixed-potential-type sensoraccording to the first aspect (1) of the invention, the second layer isformed of molten glass.

According to the mixed-potential-type sensor (4), the second layerbecomes gas impermeable without fail.

In a preferred embodiment (5) of the mixed-potential-type sensoraccording to the second aspect (2) of the invention, the second layercontains SiO₂ as a main component.

According to the mixed-potential-type sensor (5), the second layerbecomes gas impermeable without fail.

In another preferred embodiment (6) of the mixed-potential-type sensoraccording to the second aspect (2) of the invention, the second layer isformed of molten glass.

According to the mixed-potential-type sensor (6), the second layerbecomes gas impermeable without fail.

The present invention can provide a mixed-potential-type sensor whichincludes a detection electrode containing tungsten oxide, and which canprevent deterioration of the sensitivity in detecting nitrogen oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an NOx sensor apparatusincluding mixed-potential-type sensors according to an embodiment of thepresent invention;

FIG. 2 is a bottom view of a sensor unit in which a plurality ofmixed-potential-type sensors according to the embodiment of the presentinvention are connected in series;

FIG. 3 is a sectional view of the sensor unit taken along line A-A ofFIG. 2 (sectional view of one of the plurality of serially connectedmixed-potential-type sensors shown in FIG. 2);

FIG. 4 is a top view of the sensor unit including a heater;

FIGS. 5A and 5B are photographs showing an image of a cross section ofthe mixed-potential-type sensor of an example, including an electrode,observed under a scanning electron microscope, and an EPMA (electronprobe micro analyzer) image at the same position, respectively; and

FIGS. 6A and 6B are photographs showing an image of a cross section ofthe mixed-potential-type sensor of a comparative example, including anelectrode, observed under a scanning electron microscope, and an EPMA(electron probe micro analyzer) image at the same position,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference tothe drawings. However, the present invention should not be construed asbeing limited thereto.

FIG. 1 is an exploded perspective view of an NOx sensor apparatus 100which includes mixed-potential-type sensors 70 according to anembodiment of the present invention. FIG. 2 is a bottom view of a sensorunit 200 in which the plurality of mixed-potential-type sensors 70 areconnected in series. FIG. 3 is a sectional view of the sensor unit 200taken along line A-A of FIG. 2. Notably, the upper side of FIG. 1 willbe referred to as “upper side” and the lower side of FIG. 1 will bereferred to as “lower side.”

As shown in FIG. 1, the NOx sensor apparatus 100 includes the sensorunit 200, a ceramic wiring board 30 fixedly suspending the sensor unit200, a rectangular-frame-shaped first spacer 20 disposed on the upperside of the ceramic wiring board 30, a cover 10 disposed on the upperside of the spacer 20, a rectangular-frame-shaped second spacer 40disposed on the lower side of the ceramic wiring board 30, and a base 50disposed on the lower side of the second spacer 40.

The sensor unit 200 has a generally rectangular plate-like shape. Aheater 220 and a temperature sensor 221 are disposed on the uppersurface of the sensor unit 200. The plurality of mixed-potential-typesensors 70 shown in FIG. 2 are disposed on the lower surface of thesensor unit 200 and are connected in series. The sensor unit 200measures the concentration of NOx contained in a gas under measurement.

As shown in FIG. 4, conducting pads 220 a and 220 b are formed on theupper surface of the sensor unit 200 to be located near the upper ends(in FIG. 4) of left-hand and right-hand sides of the sensor unit 200.The conducting pads 220 a and 220 b form opposite ends of the heater 220which extends while meandering on the upper surface of the sensor unit200. The temperature sensor 221 extends while meandering along theheater 220 on the upper surface of the sensor unit 200. Conducting pads221 a and 221 b which form opposite ends of the temperature sensor 221are formed on the upper surface of the sensor unit 200 to be locatednear the lower ends (in FIG. 4) of the left-hand and right-hand sides ofthe sensor unit 200.

The ceramic wiring board 30 has an oblong shape and has a rectangularopening 30 h on one end side in the longitudinal direction thereof. Aplurality of lead traces 30L are formed on front and back surfaces ofthe ceramic wiring board 30. Inner ends of the lead traces 30L areconnected to a plurality of element peripheral pads 30 s surrounding theopening 30 h, and outer ends of the lead traces 30L are connected toconducting pads 30 p on the side opposite the opening 30 h in thelongitudinal direction.

The sensor unit 200 is accommodated in the opening 30 h. Four conductingmembers 30 w extend across the left-hand and right-hand sides of thesensor unit 200 and are joined to the conducting pads 220 a, 220 b, 221a and 221 b on the upper surface side of the sensor unit 200 (on theside where the heater 220 and the temperature sensor 221 are provided)and four element peripheral pads 30 s of the ceramic wiring board 30. Asa result, the sensor unit 200 is fixedly suspended within the opening 30h of the ceramic wiring board 30.

Meanwhile, as shown in FIG. 2, on the lower surface side of the sensorunit 200 (the side where the mixed-potential-type sensors 70 areprovided), end portions 206 a and 212 a of lead traces 206 and 212constitute a pair of input/output terminals (electrode pads). Althoughnot illustrated, two element peripheral pads 30 s surrounding theopening 30 h and the end portions 206 a and 212 a are joined byconducting members.

Notably, as shown in FIG. 1, on the upper surface side of the sensorunit 200, of the inner ends of the six lead traces 30L, the inner endsof the leftmost lead trace and the fourth lead trace as counted from theleft-hand side are not connected to the element peripheral pads 30 s onthe upper surface of the sensor unit 200. Rather, they are connected totwo through holes at a location near the center of the ceramic wiringboard 30.

Although not illustrated, on the lower surface side of the sensor unit200, the outer ends of the lead traces 30L connected to the elementperipheral pads 30 s on the lower surface of the sensor unit 200 areconnected to the above-mentioned lead traces on the upper surface sidethrough the above-mentioned two through holes, and are connected to theleftmost conducting pad 30 p and the fourth conducting pad 30 p ascounted from the left-hand side.

In this manner, electrical signals output from the mixed-potential-typesensors 70 and the temperature sensor 221 are output to the outsidethrough the conducting pads 30 p, and the heater 220 is energized forheat generation by electric power externally supplied through theconducting pads 30 p.

The first spacer 20 has a square shape and has a rectangular opening 20h which overlaps the opening 30 h and is larger than the opening 30 h.

The cover 10 has a square shape and has the same dimensions as the firstspacer 20. A gas discharge hole 10 h is formed in a portion of the cover10 which faces the opening 20 h.

The second spacer 40 has an oblong shape and has the same dimensions asthe ceramic wiring board 30. The second spacer 40 has a rectangularopening 40 h on the same side as the opening 30 h with respect to thelongitudinal direction. The opening 40 h overlaps the opening 30 h andis larger than the opening 30 h.

The base 50 has an oblong shape and has the same dimensions as theceramic wiring board 30. A gas introduction hole 50 h is formed in aportion of the base 50 which faces the opening 40 h.

The ceramic wiring board 30, the first spacer 20, the cover 10, thesecond spacer 40, and the base 50 may be formed of a ceramic materialsuch as alumina.

Square seals 64 and 62 are disposed between the ceramic wiring board 30and the first spacer 20 and between the first spacer 20 and the cover10, respectively, to surround the opening 20 h. Similarly, oblong seals66 and 68 are disposed between the ceramic wiring board 30 and thesecond spacer 40 and between the second spacer 40 and the base 50,respectively, to surround the opening 40 h. The seals 62 to 68 areformed of glass.

In the present embodiment, the cover 10, the first spacer 20, theceramic wiring board 30, the second spacer 40, and the base 50 areformed of a ceramic material, and are gastightly bonded and stackedtogether via the seals 62 to 68 formed of glass-based adhesive layers.

The ceramic wiring board 30 has positioning holes 30 a provided atopposite ends of an end portion thereof located on the opening 30 h sidewith respect to the longitudinal direction. Similarly, the ceramicwiring board 30 has positioning holes 30 b provided at opposite ends ofan end portion thereof located on the conducting pads 30 p side.

The first spacer 20 and the cover 10 have positioning holes 20 a and 10a, respectively, which are provided at the same positions as thepositioning holes 30 a.

Similarly, the second spacer 40 has positioning holes 40 a and 40 bprovided at the same positions as the positioning holes 30 a and 30 b,respectively, and the base 50 has positioning holes 50 a and 50 bprovided at the same positions as the positioning holes 30 a and 30 b,respectively.

The cover 10, the first spacer 20, the ceramic wiring board 30, thesecond spacer 40 and the base 50 (these members are also referred to as“the respective members”) are stacked in this order, jigs (guide pins)are passed through the positioning holes 10 a to 50 a, 40 b and 50 b tothereby position the respective members, and the respective members arebonded together, whereby the NOx sensor apparatus 100 can be assembled.

The gas under measurement introduced through the gas introduction hole50 h flows through an internal space formed by the opening 40 h, comesinto contact with the mixed-potential-type sensors 70 of the sensor unit200, by which the NOx concentration is measured, flows through aninternal space formed by the opening 20 h, and is discharged to theoutside through the gas discharge hole 10 h.

Next, the structures of the sensor unit 200 and the mixed-potential-typesensors 70 will be described with reference to FIGS. 2 and 3.

As shown in FIG. 2, the sensor unit 200 includes a generally rectangularplate-shaped base substrate 202. The plurality (9 in FIG. 2) ofmixed-potential-type sensors 70 each including a solid electrolyte layer74 and a pair of electrodes 76 and 78 provided thereon are arrayed atpredetermined intervals on the lower surface of the base substrate 202.Notably, the mixed-potential-type sensors 70 are disposed on the lowersurface of the base substrate 202 to form a 3×3 matrix; i.e., such thateach row extending in the left-right direction of FIG. 2 includes threemixed-potential-type sensors 70 and each column extending in thevertical direction includes three mixed-potential-type sensors 70.

The mixed-potential-type sensors 70 are connected in series by leadtraces 206, 208, 210 and 212. Of these traces, the lead traces 206 and212 have end portions 206 a and 212 a which serve as a pair ofinput/output terminals (electrode pads) which are the start and endpoints of the current path of the series circuit.

The heater 220 (see FIG. 4) provided on the upper surface of the basesubstrate 202 heats the mixed-potential-type sensors 70 to theiroperation temperature.

The base substrate 202 may be formed of a ceramic material such asalumina. The heater 220 and the temperature sensor 221 may be formed ofa metal such as platinum.

Meanwhile, as shown in FIG. 3, the solid electrolyte layer 74 and thetwo electrodes 76 and 78 of each mixed-potential-type sensor 70 areprovided on the lower surface of the base substrate 202. The solidelectrolyte layer 74 has a generally rectangular shape and is formed ofa porous solid electrolyte having oxygen-ion conductivity and gaspermeability.

The electrode 78 (corresponding to “the other of the porous electrodes”of the invention) which is a porous electrode extends from a positionnear one side of the solid electrolyte layer 74 toward the outside ofthe solid electrolyte layer 74 while contacting the surface of the solidelectrolyte layer 74, and is in contact with the lower surface of thebase substrate 202. The externally exposed surface of a portion of theelectrode 78, which portion is in contact with the solid electrolytelayer 74, is covered with a gas impermeable second layer 78 a.

The electrode 76 (corresponding to “one of the porous electrodes” of theinvention) which is a porous electrode extends from a position near theopposite side the solid electrolyte layer 74 (the side opposite to theelectrode 78) toward the outside of the solid electrolyte layer 74 whilecontacting the surface of the solid electrolyte layer 74, and is incontact with the lower surface of the base substrate 202. The externallyexposed surface of a portion of the electrode 76, which portion is incontact with the solid electrolyte layer 74, is covered with a firstlayer 76 a which contains tungsten oxide (WO₃) as a main component (inan amount greater than 50 mass %). Notably, as shown in FIG. 2, theelectrode 76 is formed along three sides of the solid electrolyte layer74, other than the side adjoining to the electrode 78, so as to form aU-like shape to thereby surround the electrode 78. The solid electrolytelayer 74 is exposed to the outside in a region between the electrode 78and the electrode 76.

The lead traces 206 and 208 are electrically connected to portions ofthe electrodes 76 and 78, respectively, which portions are in contactwith the lower surface of the base substrate 202.

The electrodes 76 and 78 may contain, for example, Pt as a maincomponent (in an amount greater than 50 mass %). The second layer 78 amay contain molten SiO₂ as a main component (in an amount greater than50 mass %) or may be formed of molten glass.

Each mixed-potential-type sensor 70 is formed by applying pastematerials for forming the solid electrolyte layer 74, the electrodes 76and 78, the first layer 76 a, and the second layer 78 a onto the basesubstrate 202 by, for example, printing, followed by firing. As shown inFIG. 3, as a result of the firing, a tungsten oxide component 79originating from the first layer 76 a diffuses into the electrode 76 andreaches (exists at) the interface S1 between the electrode 76 and thesolid electrolyte layer 74. As a result, the electrode 76 loses itscatalytic activity for converting NO₂ to NO and functions as a detectionelectrode for conveying NO₂ to the interface S1.

Meanwhile, as a result of the firing, the tungsten oxide component 79within the first layer 76 a sublimates (scatters). However, in themixed-potential-type sensor 70 of the present embodiment, on account ofproviding the gas impermeable second layer 78 a, the tungsten oxidecomponent 79 cannot reach the interface S2 between the second layer 78 aand the electrode 78, and the tungsten oxide component 79 is not presentat the interface S2. The tungsten oxide component 79 originating fromthe first layer 76 a is captured by the second layer 78 a and isprevented from reaching the interface S2 between the second layer 78 aand the electrode 78. As a result, the electrode 78 has a catalyticactivity for converting NO₂ to NO at a ratio corresponding to thetemperature and functions as a reference electrode.

As described above, the second layer 78 a prevents the referenceelectrode 78 from losing its catalytic activity. Therefore, a potentialdifference is reliably developed between the detection electrode 76which has no catalytic activity and conveys NO₂ to the interface S1 andthe reference electrode 78 which converts NO₂ to NO. Thus, deteriorationof the sensitivity in detecting NOx (nitrogen oxide) can be prevented.

Notably, since the electrode 78 is covered with the gas impermeablesecond layer 78 a, it becomes difficult to introduce the gas undermeasurement from the surface of the electrode 78. In view of this, thesolid electrolyte layer 74 is formed of a gas permeable porous solidelectrolyte. Therefore, as shown in FIG. 3, the gas under measurementcan flow along a route R which extends from the externally exposedsurface of the solid electrolyte layer 74 to the electrode 78 throughthe solid electrolyte layer 74, and reach the electrode 78.

Notably, the fact that the solid electrolyte layer 74 and the electrodes76 and 78 are porous can be confirmed by checking whether or not poresare preset in a secondary-electron image of a cross section of the layerand the electrodes.

The present invention is not limited to the above-described embodimentand encompasses various modifications and equivalents falling within thescope of the invention.

For example, the shapes of the solid electrolyte layer, the shape of theporous electrodes, the shape of the mixed-potential-type sensor, etc.,are not limited to those of the above-described embodiment.

EXAMPLE 1

A mixed-potential-type sensor having the structure shown in FIG. 3 wasmanufactured as follows.

First, a green base substrate 202 of alumina was formed by a doctorblade method. A Pt paste was screen-printed on one side of the greenbase substrate 202 to thereby form the heater 220 and the temperaturesensor 221. After that, the entirety was heated at 400° C. for 4 hoursfor debindering and fired at 1,350° C. for 2 hours.

A YSZ (the addition amount of Y to ZrO₂: 8 mol %) paste wasscreen-printed on the opposite side of the fired base substrate 202, andfiring was carried out at 1,350° C. for 3 hours in a nitrogen atmosphereto thereby form the solid electrolyte layer 74. In this firing,sintering of YSZ was not allowed to progress sufficiently, so that thesolid electrolyte layer 74 became porous. Another method for making thesolid electrolyte layer 74 porous is to add glass particles or the liketo the YSZ paste which foam at that firing temperature.

Subsequently, a Pt paste was screen-printed on the surface of the solidelectrolyte layer 74, and firing was carried out at 850° C. for 10minutes so as to form the electrodes 76 and 78. Notably, in order toimprove adhesion to the first layer 76 a, glass particles may be addedto the Pt paste for the electrode 76.

Next, a paste containing zeolite as a main component was screen-printedon the surface of the electrode 78, and firing was carried out at 950°C. for 2 hours so as to form the second layer 78 a. Subsequently, atungsten oxide paste was screen-printed on the surface of the electrode76, and firing was carried out at 750° C. for 1 hour so as to form thefirst layer 76 a. As a result, the mixed-potential-type sensor 70 of theexample was completed. Notably, since the second layer 78 a was formedby firing the paste containing zeolite as a main component at 950° C.for 2 hours, the second layer 78 a was formed as a gas impermeable layerof dense molten glass.

As a comparative example, a mixed-potential-type sensor was manufacturedin the same manner except that the second layer 78 a was formed byfiring zeolite at 875° C. for 1 hour.

FIGS. 5A and 5B show an image of a cross section of themixed-potential-type sensor of the example, including the electrode 78,observed under a scanning electron microscope, and an EPMA (electronprobe micro analyzer) image at the same position, respectively. FIGS. 6Aand 6B show an image of a cross section of the mixed-potential-typesensor of the comparative example, including the electrode 78, observedunder a scanning electron microscope, and an EPMA (electron probe microanalyzer) image at the same position, respectively.

In the case of the mixed-potential-type sensor of the example, as shownin FIG. 5B, the tungsten oxide component exists only on the surface ofthe second layer 78 a and cannot reach the interface S2 between thesecond layer 78 a and the electrode 78.

In contrast, in the case of the mixed-potential-type sensor of thecomparative example, as shown in FIG. 6B, the tungsten oxide componentreaches the interface S2 between the second layer 78 a and the electrode78.

The invention has been described in detail with reference to the aboveembodiments. However, the invention should not be construed as beinglimited thereto. It should further be apparent to those skilled in theart that various changes in form and detail of the invention as shownand described above may be made. It is intended that such changes beincluded within the spirit and scope of the claims appended hereto.

What is claimed is:
 1. A mixed-potential-type sensor for detecting theconcentration of nitrogen oxide contained in a gas under measurement,comprising: a solid electrolyte layer having oxygen-ion conductivity;and a pair of porous electrodes formed on the solid electrolyte layer,wherein one of the porous electrodes is covered with a first layercontaining tungsten oxide as a main component, the other of the porouselectrodes is covered with a gas impermeable second layer, the secondlayer is in contact with the other of the porous electrodes withoutintervention of a tungsten oxide component, the solid electrolyte layeris porous and allows the gas under measurement to permeate from anexternally exposed surface of the solid electrolyte layer to the otherof the porous electrodes, and the concentration of the nitrogen oxidecontained in the gas under measurement is detected from a potentialdifference developed between the porous electrodes.
 2. Amixed-potential-type sensor for detecting the concentration of nitrogenoxide contained in a gas under measurement, comprising: a solidelectrolyte layer having oxygen-ion conductivity; and a pair of porouselectrodes formed on the solid electrolyte layer, wherein one of theporous electrodes is covered with a first layer containing tungstenoxide as a main component, the other of the porous electrodes is coveredwith a second layer which captures a tungsten oxide componentoriginating from the first layer, the solid electrolyte layer is porousand allows the gas under measurement to permeate from an externallyexposed surface of the solid electrolyte layer to the other of theporous electrodes, and the concentration of the nitrogen oxide containedin the gas under measurement is detected from a potential differencedeveloped between the porous electrodes.
 3. The mixed-potential-typesensor as claimed in claim 1, wherein the second layer contains SiO₂ asa main component.
 4. The mixed-potential-type sensor as claimed in claim1, wherein the second layer is formed of molten glass.
 5. Themixed-potential-type sensor as claimed in claim 2, wherein the secondlayer contains SiO₂ as a main component.
 6. The mixed-potential-typesensor as claimed in claim 2, wherein the second layer is formed ofmolten glass.